North Korea-linked Lazarus APT targets Microsoft IIS servers to deploy malware

North Korea-linked APT group Lazarus actor has been targeting vulnerable Microsoft IIS servers to deploy malware.

AhnLab Security Emergency response Center (ASEC) researchers reported that the Lazarus APT Group is targeting vulnerable versions of Microsoft IIS servers in a recent wave of malware-based attacks.

Once discovered a vulnerable ISS server, the attackers leverage the DLL side-loading (T1574.002) technique to execute a malicious DLL (msvcr100.dll) that they have placed in the same folder path as a normal application (Wordconv.exe). Then the library is executed via the Windows IIS web server process.

The msvcr100.dll is contained within the import DLL list of Wordconv.exe, this means that the first DLL is loaded in the memory of the Wordconv.exe process when it is executed.

“the functionality of msvcr100.dll involves decrypting an encoded PE file (msvcr100.dat) and the key (df2bsr2rob5s1f8788yk6ddi4x0wz1jq) that is transmitted as a command-line argument during the execution of Wordconv.exe by utilizing the Salsa20 algorithm.” reads the analysis published by ASEC. “The decrypted PE file is then executed in the memory. It then performs the function of clearing the malicious DLL module that was loaded through the FreeLibraryAndExitThread WinAPI call before deleting itself (msvcr100.dll).”

The researchers noticed important similarities between the msvcr100.dll and the cylvc.dll previously detailed by ASEC and related to another Lazarus campaign.

The threat actor exploited an open-source Notepad++ plugin called Quick Color Picker (a discontinued project) to establish a foothold in the target network before creating additional malware (diagn.dll).

The diagn.dll received the PE file encoded with the RC6 algorithm as an execution argument value, then uses an internally hard-coded key to decrypt the data file and execute the PE file directly in the memory.

The researchers were not able to determine the malicious behavior of the PE file because the PE data file that was encoded during the attack could not be collected, but the analysis of the log suggests threat the attackers had executed a credential theft tool such as Mimikatz.

Once obtained the system credentials, the threat actor performed internal reconnaissance and used remote access (port 3389) to perform lateral movement into the internal network.

“The Lazarus group used a variety of attack vectors to perform their initial breach, including Log4Shell,  public certificate vulnerability, 3CX supply chain attack, etc.” concludes the report that also provides Indicators of Compromise (IoCs). “since the threat group primarily utilizes the DLL side-loading technique during their initial infiltrations, companies should proactively monitor abnormal process execution relationships and take preemptive measures to prevent the threat group from carrying out activities such as information exfiltration and lateral movement.”

This week, the US Department of the Treasury’s Office of Foreign Assets Control (OFAC) announced sanctions against four entities and one individual for their role in malicious cyber operations conducted to support the government of North Korea.

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